Cancer is a serious disease, which is one of the leading causes of death, but the need for treatment thereof has not yet been satisfied. In order to solve the problem of conventional chemotherapy techniques, which disadvantageously damage normal cells, in recent years, active studies have been conducted on a cancer treatment technique performed with the use of a molecular-targeting drug that has been designed to target a specific molecule expressed specifically in a cancer cell.
An example of a molecule that can be the target of a molecular-targeted drug in cancer is cadherin. Cadherin is a membrane protein which was discovered as a molecule associated with calcium-dependent, homophilic cell adhesion (Non-Patent Document 1). Proteins having cadherin repeats (ECs) composed of about 110 amino acid residues, which are highly homologous to each other, are referred to as the cadherin superfamily. The cadherin superfamily includes 120 or more protein species and plays a key role in maintenance of the multicell-layered structure.
It has been reported that cadherin expression levels are elevated in cancer cells. Accordingly, use of a drug comprising an antibody that recognizes cadherin and an anticancer agent bound thereto or an antibody having antibody-dependent cytotoxic activity (ADCC) for treatment of cancer has been studied for cancer cells exhibiting a higher cadherin expression level in cancer tissue compared with that in normal tissue (Patent Document 1: Patent Document 2).
Proteins belonging to the cadherin superfamily can be roughly classified as follows in accordance with the structural features thereof: 1) classical cadherins; 2) desmosomal cadherins; 3) protocadherins; and 4) others. Major members of the cadherin superfamily; i.e., classical cadherins such as E-cadherin (CDH1), N-cadherin (CDH2), and P-cadherin (CDH3) are highly homologous to each other (FIG. 1). Specifically, such proteins are single-pass transmembrane proteins that are presumed to form dimers, and have 5 cadherin extracellular domains (EC1 to EC5) and intracellular domains. Cell adhesion mediated by classical cadherins are characterized by adhesion between homogeneous cells, and such cell adhesion takes place when the cells recognizes the cadherin molecule of the same species that are expressed specifically and differently depending on cell species. Specifically, CDH1 recognizes and binds to CDH1, and CDH3 recognizes and binds to CDH3. Thus, cells of the same species adhere to each other (FIG. 2).
Homologous/heterologous cadherins are deduced to be recognized by the cadherin domain 1 (EC1) located at the N terminus of the extracellular domain (Non-Patent Document 2). Klingel et al. demonstrate that, even when a sequence comprising positions 1 to 213 of the amino acid sequence of human CDH3 (SEQ ID NO: 2) is substituted with the corresponding domain of human CDH1, it would bind to CDH3 instead of CDH1 (Non-Patent Document 3). Thus, classical cadherins, including CDH1 and CDH3, are considered to bind each other based on the same mechanism.
In recent years, many antibody drugs for cancer treatment have actually been marketed as molecular-targeted drugs, and many of such drugs are based on the ADCC mechanism. However, the drug efficacy thereof is not always sufficient, and development of techniques exerting more potent anti-tumor effects has been attempted.
An example of an effective means for potentiating the antitumor activity of the antibody is a conjugate of an antibody and a highly toxic drug (toxin). If a toxin is administered to a patient by itself, disadvantageously, it damages normal tissues. Accordingly, it cannot serve as an effective therapeutic means. By binding a toxin to an antibody that binds to a tumor-cell-specific antigen, however, a toxin can selectively destroy tumor cells without adversely affecting normal tissue. Such drugs are referred to as “antibody-drug conjugates (ADCs).” Specifically, a toxin does not exert its toxicity while being bound to an antibody. However, some antibodies are incorporated into cells when the antibodys are bound to an target antigen, and are then degraded in lysosomes. Accordingly, such antibodies comprising toxins bound thereto are incorporated and degraded in cells, toxins are released, toxicity is expressed selectively in specific cells, and cells are destroyed thereby.
In ADCs, drugs bound to antibodies circulate in the blood, and such drugs accumulate and exert drug efficacy in target tumor cells. It is not preferable that a drug be released at any site other than tumor regions (i.e., liberation from antibodies) due to a risk of adverse side effects. That is, it is preferable that ADCs be designed in such a manner that drugs bound to antibodies are first incorporated into cells and then liberated from the antibodies. From such point of view, Genentech Inc. have developed drugs comprising Trastuzumab and toxin bound thereto (T-DM1), the developed drugs have been subjected to clinical testing, and such drugs exert remarkably high clinical effects. That is, it is not sufficient if ADCs merely accumulate in target cancer cells, and it is necessary that ADCs are efficiently incorporated into cancer cells. Such capacity (an internalization capacity of an antibody) is closely related to drug efficacy of ADC.
The internalization capacity of an antibody is affected by both a membrane surface protein to which an antibody binds and an antibody itself. Accordingly, such capacity cannot be unambiguously deduced based on molecular structure, physical properties of antibody, or other factors. Screening of relevant antibodies with a high internalization capacity against antigens is accordingly a big object in the development of ADCs. The present invention is intended to overcome such object with respect to the CDH3 antigen.
As described above, the concept of cancer treatment with the use of ADCs has been known. However, none of such techniques suggest the efficacy of an immunocomplex resulting from conjugation of an anti-cadherin antibody having a high internalization capacity with a drug. While the correlation between a domain structure and a cadherin function has been reported, no reports have been made regarding the correlation between a domain structure and an internalization capacity.